As per the error "RPC unavailable" it can be that the storage system is unable to communicate with the host over RPC. You can try to change the communication protocol for that storage system to HTTP/ HTTPs with the correct authentication credentials and see if it enumerates the LUNs. This can be done from the "Change transport protocol settings " option on the SDW MMC.
I've experienced this in Windows 2008R2 when the firewall service is running (protection could be disabled but the service itself is still active). I've also experienced this when the server and array are separated by a firewall (like a PCI compliant zone). I'd recommend using a tool like NMAP to look at open ports. As a test, stop the firewall service from the Windows 2008R2 server and also try changing the snapdrive configuration to use HTTP or HTTPS - obviously, don't leave the configuration at HTTP or the firewall disabled but you can use it to troubleshoot.
Also, standard credential stuff:
Ensure that the user id is a local administrator
Make sure the ID has appropriate access to the filer (administrator or specific grants for RBAC)
I've seen some oddities using credentials across trusted domains - easiest way around this is to configure as passthrough account (same ID / password on the local SAM of the filer)